1. Field of the Invention
The present invention relates to an electromechanical transducer that transduces an electric signal into mechanical vibration and an electroacoustic transducer that transduces an electric signal into sound, and particularly relates to an electromechanical transducer and an electroacoustic transducer that comprise a driving unit including an armature, a yoke, a coil and magnets.
2. Description of Related Art
An electroacoustic transducer used in a hearing aid or the like is provided with a driving unit including an armature, a yoke, a coil and a pair of magnets, etc., and is configured to drive the armature in response to an electric signal supplied to the coil so that relative vibration between the armature and other members is transduced into sound. For example, as examples of a so-called balanced armature type (hereinafter, referred to as “balanced type”) electromagnetic transducer, Patent Reference 1 discloses a structural example of a magnetic circuit using a U-shaped armature, and Patent Reference 2 discloses a structural example of a magnetic circuit using an E-shaped armature. Patent Reference 3 discloses a structural example for improving shock resistance in an electroacoustic transducer provided with a magnetic circuit of the above-mentioned balanced type. In any of these structural examples, the armature is connected to the yoke so as to form the magnetic circuit. Further, in any of the above cases, the armature needs to be formed so that restoring force due to elasticity of the armature itself when one end thereof is displaced is larger than magnetic forces (attraction) of the magnets that are generated by the displacement of the armature. In case of employing the structures disclosed in the Patent References 1 to 3, it is essential to form the armature using soft magnetic material.    [Patent Reference 1] U.S. Pat. No. 7,869,610    [Patent Reference 2] U.S. Pat. No. 4,473,722    [Patent Reference 3] Japanese Patent Application Laid-open No. 2006-041768
However, in the above conventional balanced type electromagnetic transducer, since the armature constitutes a part of the magnetic circuit, it is required to satisfy design requirements for magnetic property of the magnetic circuit. Although the armature is required to be designed so that the restoring force caused by displacement of the armature is larger than the magnetic forces of the magnets, as described above, the armature of the conventional structure is required to be designed so as to satisfy both magnetic and mechanical requirements because the armature gets the restoring force due to the elasticity of its own. For example, in order to improve the shock resistance of the armature, an option is to increase thickness of the armature. However, if the thickness of the armature is increased, it is not possible to obtain sufficient amount of displacement within an elasticity range of the armature. Further, since the armature is required to have necessary magnetic property, it is difficult to use general spring material having large yield stress and strong shock resistance. In general, magnetic annealing treatment is performed for the armature after forming process for the purpose of extracting the magnetic property of its material, and therefore it is also difficult to increase the yield stress of the armature by heat treatment. As described above, according to the conventional structure, it is inevitable that degree of freedom in designing the armature is largely restricted.